1. Field of the Invention
The present invention relates to a method for manufacturing p-type and n-type thin film transistors, and particularly to a method for manufacturing a thin film transistor through a laser radiation process.
2. Description of the Related Art
Conventionally, there are two primary methods for irradiating amorphous silicon with an Excimer laser (EL) beam to anneal the amorphous silicon, converting the amorphous silicon into polycrystalline silicon. One of the two primary methods includes converting amorphous silicon into polycrystalline silicon without previously implanting dopant atoms into the amorphous silicon (non-doped amorphous silicon). In contrast, the other of the two primary methods includes previously doping dopant atoms into amorphous silicon to convert the amorphous silicon into of an N-type amorphous silicon layer (for formation of P-channel transistor) or a P-type amorphous silicon layer (for formation of N-channel transistor) and irradiating the amorphous silicon layer with an Excimer laser beam to convert the amorphous silicon of the silicon layer into polycrystalline silicon.
Generally, when doping dopant atoms used as a solid phase diffusion source into amorphous silicon, a speed at which amorphous silicon is crystallized when converting amorphous silicon into polycrystalline silicon increases. However, employment of the aforementioned method produces the following results. That is, when an amorphous silicon film is doped with dopant atoms to convert the amorphous silicon film into an N-type amorphous silicon layer (for formation of P-channel transistor) and the amorphous silicon of the silicon layer is converted into polycrystalline silicon by irradiation of Excimer laser beam, the threshold voltage of P-channel transistor can be adjusted, but the threshold voltage of N-channel transistor cannot be adjusted. That is, employment of the conventional method gives wide variations in the threshold voltage of N-channel transistor. Accordingly, it can be concluded that although a P-channel transistor is formed to have threshold voltage levels within a specific range of values, a P-channel transistor and an N-channel transistor are formed to have threshold voltages at levels different from each other due to the variation of N-channel transistor threshold voltage, incurring a substantial processing burden on the design of circuit.